Industrial production of continuous web materials often requires measurement of caliper or basis weight (weight per area unit) for testing and control purposes. For example, in the case of manufacturing coatings, co-extruded plastics, and related multi-layer products, it is often necessary to measure the caliper or basis weight of each of the layers and/or coats. Standard means for determining the caliper of continuous web coatings include, for instance, differential weigh techniques that rely on nuclear or x-ray measurements. This non-contacting scheme employs dual scanners, each equipped with a sensor, that measure the web weight before and after the coating procedure, with the difference being the coating weight. An analogous dual scanner method is the contacting, differential caliper measurement where subtraction of the web caliper before and after the coating procedure yields the coating weight. Finally, infrared radiation detection uses a distinctive IR signature of the coating to detect and measure coating weight.
The utility of these prior art caliper measurement techniques is restricted by a number of drawbacks. Two-scanner differential systems are prohibitively expensive and require excessive space for accommodating both scanners. Furthermore, the technique itself is not reliable for detecting relatively thin coatings that are applied onto a thick base material. Indeed, dual-scanner measurements often yield calculated coating thicknesses with significant errors. Variability of the base layer is another source of error in dual-scanner systems. To compensate for this variability, dual-scanner systems often include same-spot software that compares measurement results obtained by both scanners from the same spots on the moving web. Unfortunately, the scanners do not remain perfectly aligned and their inability to follow the same spot renders the measurement useless especially when the scanners are measuring a traveling web whose thickness is not uniform in the machine direction. It has been demonstrated that conventional dual scanner systems when measuring a thin polymer coating on a baseboard can exhibit an error that is larger than the measured thickness of the coating. Finally, IR sensing is sometimes impossible to implement due to the high IR attenuation in the measured material or lack of distinctive IR signatures that would allow for differentiation of the coating material. The art is in need of an accurate and repeatable technique for measuring the thickness and related properties of coatings and films that are formed on continuous, traveling non-uniform webs.